U.S. patent number 6,024,856 [Application Number 08/948,748] was granted by the patent office on 2000-02-15 for copper metallization of silicon wafers using insoluble anodes.
This patent grant is currently assigned to Enthone-OMI, Inc.. Invention is credited to Juan B. Haydu, Richard W. Hurtubise, Elena H. Too.
United States Patent |
6,024,856 |
Haydu , et al. |
February 15, 2000 |
Copper metallization of silicon wafers using insoluble anodes
Abstract
A plating system and method is provided for electroplating
silicon wafers with copper using an insoluble anode wherein the
electrolyte is agitated or preferably circulated through an
electroplating tank of the system and a portion of the electrolyte
is removed from the system when a predetermined operating parameter
is met. A copper containing solution having a copper concentration
greater than the copper concentration of the removed portion is
added to the copper plating system simultaneously or after
electrolyte removal, in a substantially equal amount to the
electrolyte removed from the system and balances the amount of
copper plated and removed in the removal stream. In a preferred
method and system, an electrolyte holding tank is provided which
serves as a reservoir for circulating electrolyte. The addition of
the copper containing solution and removal of working electrolyte
is also preferably made from the holding tank. The preferred
apparatus is preferably cylindrical and is specially configured so
that recirculating electrolyte enters near the anode and exits near
the cathode with the outlet of the apparatus having a substantially
continuous opening around the periphery of the electroplating tank
so that electrolyte exiting the tank has a substantially uniform
flow across the surface of the cathode. The anode is preferably
circular and has a central through opening through which the
entering electrolyte passes.
Inventors: |
Haydu; Juan B. (Orange, CT),
Too; Elena H. (Branford, CT), Hurtubise; Richard W.
(Clinton, CT) |
Assignee: |
Enthone-OMI, Inc. (West Haven,
CT)
|
Family
ID: |
25488219 |
Appl.
No.: |
08/948,748 |
Filed: |
October 10, 1997 |
Current U.S.
Class: |
205/84;
257/E21.175; 204/232; 204/237; 205/101 |
Current CPC
Class: |
C25D
21/14 (20130101); C25D 21/18 (20130101); H01L
21/2885 (20130101); C25D 17/001 (20130101); C25D
7/123 (20130101) |
Current International
Class: |
C25D
21/14 (20060101); C25D 21/12 (20060101); C25D
021/14 (); C25D 021/18 () |
Field of
Search: |
;205/82,84,101,232,237,238,239,240,241 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
H36 |
March 1986 |
Smith |
4469564 |
September 1984 |
Okinaka et al. |
4778572 |
October 1988 |
Brown |
5000827 |
March 1991 |
Schuster et al. |
5100517 |
March 1992 |
Starinshak et al. |
5143593 |
September 1992 |
Ueno et al. |
5344491 |
September 1994 |
Katoh |
5352350 |
October 1994 |
Andricacos et al. |
5368711 |
November 1994 |
Poris |
5368715 |
November 1994 |
Hurley et al. |
5385661 |
January 1995 |
Andricacos et al. |
5391517 |
February 1995 |
Gelatos et al. |
5516414 |
May 1996 |
Glafenhein et al. |
|
Foreign Patent Documents
|
|
|
|
|
|
|
0 508 212 B1 |
|
Oct 1995 |
|
GB |
|
0 462 943 B1 |
|
Oct 1996 |
|
GB |
|
Other References
Wire Journal International, "Copper Plating Insoluble Anode System
for Steel Cord Process: Development of Copper Ion Supply System",
Aug. 1997, pp. 82-87..
|
Primary Examiner: Gorgos; Kathryn
Assistant Examiner: Leader; William T.
Attorney, Agent or Firm: DeLio & Peterson LLC Mueller;
Richard P.
Claims
Thus, having described the invention, what is claimed is:
1. An electrolytic method for copper plating a silicon wafer
semiconductor substrate wherein electrolyte bath components are
non-accumulating and reach a steady-state value comprising the
steps of:
providing a plating system comprising a copper plating apparatus
comprising a tank and containing a copper electrolyte comprising
copper and an additive system for enhancing copper plating
properties and a substrate to be plated as a cathode and a spaced
apart insoluble anode;
agitating the copper electrolyte in the tank, while applying a
current and electroplating the cathode;
measuring the weight of copper plated;
removing a portion of the electrolyte from the system when a
predetermined weight of copper plated is met;
adding to the system either simultaneously with or after the
electrolyte removal so that the volume of electrolyte in the system
is substantially constant a replenishment copper and additive
system containing solution, the solution having a copper
concentration greater than the copper concentration in the removed
electrolyte and an additive concentration greater than the additive
concentration in the removed electrolyte and in an amount
sufficient to increase the copper concentration and additive
concentration of the electrolyte in the system to a predetermined
value by replacing the copper and additive used in the plating
process and removed in the removal portion; and
maintaining the volume of electrolyte in the system at a
substantially constant volume during the electroplating.
2. The method of claim 1 wherein the plating system comprises:
a copper plating tank and an electrolyte holding tank, the copper
plating tank being cylindrical and having an inlet at the lower end
of the tank and an outlet at the upper end of the tank and
containing copper electrolyte and a substrate as a cathode and a
spaced apart insoluble anode having a central opening therein both
the cathode and anode being horizontal, the inlet and outlet
positioned so that an electrolyte stream enters the plating tank
proximate the anode and flows through the central opening toward
the cathode and exits proximate the cathode; and
agitating the electrolyte by circulating the copper electrolyte to
the inlet of the apparatus from the holding tank and simultaneously
removing the copper electrolyte from the outlet of the apparatus
and directing the outlet copper electrolyte to the holding
tank.
3. The method of claim 2 wherein the electrolyte and copper
containing solution comprises copper sulfate and the copper
containing solution contains copper in an amount of about 70-80
g/l.
4. The method of claim 3 wherein the anode to cathode ratio is
about 1:1 to 4:1.
5. The method of claim 4 wherein the residence time of electrolyte
in the copper plating tank is less than 1 minute.
6. The method of claim 1 wherein the copper containing solution is
added using a single use container which contains the same amount
of solution as the amount removed in the removal portion.
7. An apparatus for electrolytic copper plating a silicon wafer
semiconductor substrate wherein electrolyte bath components are
non-accumulating and reach a steady-state value, comprising:
an electroplating tank containing therein copper electrolyte
comprising copper and an additive system for enhancing copper
plating properties, a cathode adapted to support a substrate and a
spaced apart insoluble anode, the tank having inlet means and
outlet means;
agitating means for agitating the copper electrolyte in the tank by
circulating the copper electrolyte through the electroplating tank
from the inlet to the outlet, while applying a current and
electroplating the cathode;
measuring means for measuring the weight of copper plated;
removal means for removing a portion of the electrolyte from the
electroplating tank when a predetermined weight of copper is met;
and
addition means for adding to the electroplating tank either
simultaneously with or after the electrolyte removal so that the
volume of electrolyte is substantially constant a replenishment
copper and additive system containing solution, the solution having
a copper concentration greater than the copper concentration of the
removed electrolyte and an additive concentration greater than the
additive concentration in the removed electrolyte in an amount
sufficient to increase the copper concentration and additive
concentration of the electrolyte in the system to a predetermined
value by replacing the copper and additive used in the plating
process and removed in the removal portion and to maintain the
volume of electrolyte in the tank at a substantially constant
volume during the electroplating.
8. The apparatus of claim 7 further comprising an electrolyte
holding tank from which electrolyte is pumped into an inlet of said
inlet means of the electroplating tank and into which electrolyte
is pumped from an outlet of said outlet means of the electroplating
tank.
9. The apparatus of claim 8 wherein the anode is proximate the
inlet and has an opening therein through which the circulating
electrolyte passes.
10. The apparatus of claim 9 wherein the outlet means is proximate
the cathode edge.
11. The apparatus of claim 10 wherein the outlet is a continuous
opening around the periphery of the electroplating tank.
12. The apparatus of claim 7 wherein the anode has a central
opening therein so that electrolyte entering the apparatus flows
through the opening toward the cathode.
13. The apparatus of claim 7 wherein the addition means comprises a
single use container containing the copper and additive solution.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and system for copper plating
substrates and, in particular, to the electrolytic copper
metallization of silicon wafers using insoluble anodes.
2. Description of Related Art
The demand for manufacturing semiconductor integrated circuit (IC)
devices such as computer chips with high circuit speed, high
packing density and low power dissipation requires the downward
scaling of feature sizes in ultra-large-scale integration (ULSI)
and very-large-scale integration (VLSI) structures. The trend to
smaller chip sizes and increased circuit density requires the
miniaturization of interconnect features which severely penalizes
the overall performance of the structure because of increasing
interconnect resistance and reliability concerns such as
fabrication of the interconnects and electromigration.
At present, such structures use aluminum and aluminum alloys as the
metallization on silicon wafers with silicon dioxide (SiO.sub.2)
being the dielectric material. In general, openings are formed in
the SiO.sub.2 dielectric layer in the shape of vias and trenches
which are then metallized forming the interconnects. Increased
miniaturization is reducing the openings to submicron sizes (e.g.,
0.5 micron and lower).
To achieve further miniaturization of the device, however, it is
being proposed to use copper instead of aluminum as the metal to
form the connection lines and interconnects in the chip. Copper has
a lower resistivity than aluminum and the thickness of a copper
line for the same resistance can be thinner than that of an
aluminum line. Copper-based interconnects will therefore represent
the future trend in the fabrication of such devices.
Copper can be deposited on substrates by plating (such as
electroless and electrolytic), sputtering, plasma vapor deposition
(PVD) and chemical vapor deposition (CVD). It is generally
recognized that a plating-based deposition is the best method to
apply copper to the device since it can provide high deposition
rates and low tool costs. However, plating methods must meet the
stringent requirements of the semiconductor industry. For example,
the copper deposits must be uniform and capable of filling the
extremely small trenches and vias of the device. The plating
process must also be capable of being controlled so that plating
upsets are minimized or avoided and must also be compatible with
clean room operations. The deposition of copper from acid copper
baths is recognized in the electronics industry as the leading
candidate to copper plate integrated circuit devices.
Copper electroplating, in general, involves deposition of a copper
layer onto a surface by means of electrolysis using a consumable
copper electrode or an insoluble anode. In the consumable
electrolytic plating process, the copper electrode is consumed
during the plating operation and must be replaced periodically
during the plating operation. When plating using insoluble anodes,
these anodes are not consumed in the plating process and do not
have to be replaced. The following description will be directed to
the electrolytic plating of copper using insoluble anodes.
Regardless of the method used to deposit copper on the substrate
surface impurities may be co-deposited with the copper. In
integrated circuit fabrication it is important that impurity
particles not be present in the electrolyte but such impurities may
result from sludges formed during the plating operation, impure
chemicals and the like. As in all processes used to fabricate
integrated circuit (IC) devices, it is necessary that such
impurities be minimized and most operations are carried out in a
clean room. A clean room is basically a room in which the different
process steps are performed and dust particles and other impurity
particles are maintained below certain levels by the use of filters
and other such cleaning devices. It is important that any plating
process used for the fabrication of integrated circuit devices be
adaptable for use in a clean room and that the process itself
minimize the impurity problems inherent in the plating process.
Bearing in mind the problems and deficiencies of the prior art, it
is therefore an object of the present invention to provide an
improved method and apparatus (system) for electroplating a
substrate.
It is a further object of the present invention to provide an
improved electrolytic method and apparatus for copper plating a
silicon wafer in integrated circuit fabrication using an insoluble
anode.
It is another object of the present invention to provide an
electrolytic method and apparatus for copper plating a substrate
including silicon wafer substrates using an insoluble anode which
plating may be performed in a clean room.
It is an additional object of the invention to provide an
electrolyte copper plating process having a substantially steady
state electrolyte wherein the plating properties of the deposit
remain constant.
Another object of the invention is to provide semiconductors and
other devices electroplated with copper.
Other objects and advantages of the invention will in part be
readily apparent from the following description.
SUMMARY OF THE INVENTION
Applicants have discovered an electrolytic method for copper
plating a substrate, preferably a silicon wafer semiconductor
substrate, comprising:
providing a plating system comprising a copper plating apparatus
comprising a tank preferably having an inlet and an outlet and
containing a copper electrolyte and a substrate to be plated as a
cathode and a spaced apart insoluble anode;
agitating the copper electrolyte in the tank preferably by
circulating the electrolyte to the inlet of the tank and
simultaneously removing the copper electrolyte from the outlet of
the tank, while applying a current and electroplating the
cathode;
measuring at least one of the operating parameters of the plating
system;
removing a portion of the electrolyte from the system when a
predetermined operating parameter is met;
adding to the system either simultaneously with or after the
electrolyte removal a copper containing solution having a copper
concentration greater than the copper concentration in the removed
electrolyte and in an amount sufficient to increase the copper
concentration of the electrolyte in the system to a predetermined
value and to maintain the volume of electrolyte in the system at a
substantially constant volume.
In another aspect of the invention, a method is provided for the
electrolytic copper plating of a substrate, preferably a silicon
wafer semiconductor substrate, comprising:
providing a plating system comprising:
a copper plating tank and an electrolyte holding tank, the copper
plating tank preferably being cylindrical and having an inlet
preferably at the lower end of the tank and an outlet at the upper
end of the tank and containing copper electrolyte and a substrate
as a cathode and a spaced apart insoluble anode both the cathode
and anode being preferably horizontal, the inlet and outlet
preferably positioned so that an electrolyte stream enters the
plating tank near the anode and exits near the cathode;
agitating the electrolyte preferably by circulating the copper
electrolyte to the inlet of the apparatus from the holding tank and
simultaneously removing the copper electrolyte from the outlet of
the apparatus and directing the outlet copper electrolyte to the
holding tank, while applying a current and electroplating the
cathode;
measuring at least one of the operating parameters of the plating
system;
removing a portion of electrolyte from the system when a
predetermined operating parameter is met;
adding to the system either simultaneously with or after the
electrolyte removal a copper containing solution having a copper
concentration greater than the copper concentration in the removed
electrolyte and in an amount sufficient to increase the copper
concentration of the electrolyte in the system to a predetermined
value and to maintain the volume of electrolyte in the system at a
substantially constant volume.
In another aspect of the invention, an apparatus for electrolytic
copper plating a substrate, preferably a silicon wafer,
comprises:
an electroplating tank containing therein copper electrolyte, a
substrate as a cathode and a spaced apart insoluble anode, the tank
preferably having inlet means and outlet means;
agitating the copper electrolyte in the tank preferably by
circulating the copper electrolyte through the electroplating tank
from the inlet to the outlet, while applying a current and
electroplating the cathode;
measuring means for measuring at least one of the operating
parameters of the plating process;
removal means for removing the electrolyte from the electroplating
tank when a predetermined operating parameter is met; and
addition means for adding to the electroplating tank a copper
containing solution having a copper concentration greater than the
copper concentration of the removed electrolyte in an amount
sufficient to increase the copper concentration of the electrolyte
in the system to a predetermined value and to maintain the volume
of electrolyte in the tank at a substantially constant volume.
In a preferred embodiment, the apparatus further comprises
filtering means to filter the electrolyte and cooling/heating means
to adjust the temperature of the electrolyte. It is preferred to
filter the electrolyte as it enters the electroplating tank.
In another aspect of the invention, the apparatus further comprises
a electrolyte holding tank from which electrolyte is pumped into
the inlet of the electroplating tank and into which electrolyte is
pumped from the outlet of the electroplating tank. The addition
means to add the copper containing solution preferably adds the
solution to the holding tank. Electrolyte removed from the system
is also preferably removed from the holding tank.
In another aspect of the invention, the additive copper containing
solution is packaged in a container holding a predetermined amount
of solution to be added to the system and is added by means
suitable for clean room operations, e.g., injecting the solution
into the holding tank from the container, and the container either
discarded or available for refilling and reuse. Likewise, the
electrolyte which is removed from the system is preferably removed
into a waste container under conditions suitable for clean room
operations and which container may then be removed from the system
and the contents treated outside the clean room.
BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects of the present invention can be best understood by
reference to the accompanying drawings in which:
FIG. 1 is a schematic illustration of a copper electroplating
apparatus of the invention using a holding tank for recycle of the
copper electrolyte.
FIG. 2 is a schematic illustration of another copper electroplating
apparatus of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
In describing the preferred embodiment of the present invention,
reference will be made herein to FIGS. 1-2 of the drawings in which
like numerals refer to like features of the invention. Features of
the invention are not necessarily shown to scale in the
drawings.
Referring to FIG. 1, a plating system of the invention is shown
generally as 10 and is used for electroplating copper onto a
substrate 12. The plating system 10 and method are described with
reference to plating a silicon wafer using an insoluble anode but
it will be appreciated by those skilled in the art that other
substrates may be plated.
The preferred plating system 10 comprises an electroplating tank 11
which holds copper electrolyte 27 and which is made of a suitable
material such as plastic or other material inert to the
electrolytic plating solution. The tank is preferably cylindrical
especially for wafer plating. A cathode 12 is horizontally disposed
at the upper part of tank 11 and may be any type substrate such as
a silicon wafer having openings such as trenches and vias. The
wafer substrate 12a is typically coated with a seed layer of copper
or other metal to initiate plating thereon. A copper seed layer may
be applied by CVP, PVD and the like. An anode 13 is also preferably
circular for wafer plating and is horizontally disposed at the
lower part of tank 11 forming a space between the anode 13 and
cathode 12. The anode 13 is an insoluble anode which is not
consumed in the process. Suitable insoluble anodes include platinum
and platinum metals including platinized titanium and platinized
niobium and metal oxides, e.g., iridium oxide, ruthenium oxide,
etc. coated on a substrate such as titanium.
The cathode substrate 12 and anode 13 are electrically connected by
wiring 14 and 15, respectively, to a rectifier (power supply) 16.
The cathode substrate 12 for direct current has a negative charge
so that copper ions in the solution are reduced at the cathode
substrate forming plated copper metal on the cathode surface 12a.
An oxidation reaction takes place at anode 13 forming oxygen which
migrates from the surface of the anode in the form of bubbles which
rise in tank 11. The cathode 12 and anode 13 are shown horizontally
disposed but may also be vertically disposed in the tank 11.
An electrolyte holding tank 19 contains copper electrolyte 27 which
is recycled from holding tank 19 through line 17a, filter 26 and
line 17b to the inlet 11a of electroplating tank 11. The
electrolyte 27 as it enters the tank moves through an opening 13a
in anode 13 and moves as shown by arrows A upward to the outlets
11b and 11b' of electroplating tank 11. The anode is positioned on
plate 31. Arrows B show electrolyte being removed from holding tank
11 through outlets 11b and 11b' into recycle transfer lines 18a and
18b. It is preferred that outlets 11b and 11b' be proximate the
edge of surface 12a of cathode 12 and more preferred that the
outlet be a continuous opening around the periphery of the
electroplating tank so that the flow of electrolyte impinging on
the cathode surface is uniform across the cathode surface and the
electrolyte overflows the opening and is directed to holding tank
19 for recycle. The electrolyte thus flows through the opening 13a
in anode 13 and flows upward through tank 11 and impinges on
cathode 12 as it exits the tank 11. A flange or plate 30 holds
cathode 12 in position. As shown in the figure, electrolyte
contacts only the upper side of anode 13 and only the lower side
12a of cathode 12. The outlet electrolyte is recycled to holding
tank 19. During operation of the plating system to plate cathode
substrate 12 with a layer of copper, the electrolyte 27 is
preferably continuously recycled through holding tank 19 and
electroplating tank 11. This forms a substantially uniform
electrolyte composition in the system and contributes to the
overall effectiveness of the substrate plating.
The copper electroplating bath may vary widely depending on the
substrate to be plated and the type copper deposit desired. An acid
bath is preferred and an exemplary copper plating bath because of
its demonstrated effectiveness has a copper ion concentration of
about 15 to 19 g/l and a copper sulfate concentration as the
pentahydrate of 59 to 75 g/l. Sulfuric acid is present in an amount
of about 150 to 225 g/l. Chloride ion may also be used in the bath
at a level up to 90 mg/l. The bath also preferably contains an
additive system for brightening, ductility and other copper plate
properties.
During operation of the electroplating system 10, copper metal is
plated on surface 12a of cathode substrate 12 when the rectifier 16
is energized. A pulse current, direct current, reverse periodic
current or other suitable current may be employed. The
electroplating process results in a depletion in the copper
concentration of the copper electrolyte 27. The temperature of the
electrolyte may be maintained using a heater/cooler 22 whereby
electrolyte 27 is removed from holding tank 19 and flows through
line 23, heater/cooler 22 and then recycled to holding tank 19
through line 24.
It is an important feature of the invention that the plating system
and method of the invention be controlled by removing a portion of
the electrolyte from the system when a predetermined operating
parameter (condition) is met and new electrolyte is added to the
system either simultaneously or after the removal in substantially
the same amount. The new electrolyte is preferably a single liquid
containing all the materials needed to maintain the electroplating
bath and system. The addition/removal system of the invention
maintains a steady-state constant plating system having enhanced
plating effects such as constant plating properties. It has been
found using the system and method of the invention that the plating
bath reaches a steady state where bath components are substantially
non-accumulating, e.g., reach a steady state value such as the
sulfate concentration. The electrolyte is added as a copper
containing solution having a copper concentration greater than the
copper concentration of the electrolyte removed from the system and
increases the copper concentration in the electrolyte in the system
to a predetermined value, typically the initial copper value and/or
the copper value to be maintained. This removal and addition is
accomplished by removing electrolyte 27 which is essentially
homogenous from tank 19 through line 29 to container or tank 21. A
copper containing solution is added to the holding tank 19 through
line 28 from container or tank 20.
In one mode of operation, the electrolyte is removed and the copper
containing solution added based on the operating parameter of the
amount of copper plated in the system since the last
removal/addition procedure. This can be determined in a number of
ways such as amp-hours, copper weight, etc. In any event, when a
certain amount of copper is plated, it is preferred that a certain
amount of electrolyte be removed from the system and an equal
amount of copper containing solution added to the system. It is
preferred that the amount of copper added in the copper containing
solution be equal to the amount plated on the substrate plus the
amount removed in the removed stream. This will maintain the copper
concentration within a range which can be controlled within certain
limits depending on the plating characteristics desired. It is
preferred that the concentration of copper in the electrolyte be
maintained within about 3 g/l, preferably 2 g/l and most preferably
1 g/l or less of the desired copper concentration for wafer plating
processes.
The copper used to make the electrolyte and the copper containing
solution is preferably copper sulfate. It has been found that when
the system and method of the invention is used with copper sulfate
and a sulfate containing additive that the sulfate concentration of
the bath is maintained at an effective operating level throughout
the plating operation and the sulfate concentration does not need
to be separately measured or controlled.
The copper containing solution will also contain an additive system
which is the same additive system used in the electrolyte. The
amount of additive which is used during the plating operation can
be empirically determined and the value decreases over time during
plating. It has also been found that using the system and method of
the invention, when the amount of additive added in the copper
containing solution is substantially equal to the amount of
additive calculated to be used during the plating process plus the
amount removed from the system in the removal stream, the additive
level is maintained at the desired concentration or range in the
electrolyte without the need for separate measurement or other
process control. Accordingly, it is not necessary to measure the
amount of additive in the electrolyte or perform other analytical
measurements on the additive when using the method and plating
system of the invention.
Referring now to FIG. 2 which shows another plating system 10 of
the invention, the plating system 10 is similar to the plating
system of FIG. 1 except that a holding tank 19 is not employed.
Thus, an electroplating tank 11 has therein a horizontally disposed
cathode 12 and anode 13 separated by a space. Electrolyte 27 in the
tank is circulated through the tank and removed through outlet
lines 18a and 18b. The outlet from the tank is recycled to the
inlet of the tank through line 17a, filter 26 and line 17b into
tank 11 at inlet 11a. The flow of electrolyte 27 into the tank is
shown by arrows A and electrolyte flow to outlets 11b and 11b' past
cathode 12 as shown by arrows B. Anode 13 has a central opening
13a.
When a predetermined operating parameter is reached, electrolyte 27
is removed from the apparatus through line 29 into tank or
container 21 and a copper containing solution in tank 20 is fed
into outlet line 18a through line 28. A heater or cooler 22 is
shown employed in line 18a.
Preferred insoluble anodes include platinum and platinum metal
surfaces and oxides of iridium on a substrate such as titanium.
Generally, these anodes are made by coating these compounds on a
conducting substrate such as a titanium substrate. Other anodes may
also be used in practice of the invention and comprise generally a
Group VIII metal. Group VIII metals include cobalt, nickel,
ruthenium, rhodium, palladium, iridium and platinum.
The invention may be practiced using a large variety of copper
baths. The electrolytic baths include acid baths and alkaline
baths. A variety of copper electroplating baths are described in
the book entitled Modern Electroplating, edited by F. A. Lowenheim,
John Reily & Sons, Inc., 1974, pages 183-203. Exemplary baths
include copper fluoborate, copper pyrophosphate, copper cyanide,
copper phosphonate and other copper metal chelates such as methane
sulfonic acid and the preferred copper electroplating bath
comprises copper sulfate in an acid solution. The concentration of
copper and acid may vary over wide limits. For copper or copper
ions, compositions generally vary up to 25 g/l or more preferably
15 to 20 g/l. The acid solution is typically sulfuric acid in an
amount up to about 300 g/l or more, preferably 150 to 200 g/l.
Chloride ions may be used in the bath at levels up to about 90
mg/l.
A large variety of additives are typically used in the bath to
provide desired surface finishes for the copper plated metal.
Usually more than one additive is used with each additive forming a
desired function. The additives are generally used for improved
metal plated appearance (brightness), ductility, structure and
physical properties such as electrical conductivity. Particular
additives (usually organic additives) are used for grain
refinement, suppression of dendritic growth and improved covering
and throwing power. Typical additives used in electroplating are
discussed in a number of references including Modern
Electroplating, cited above. A particularly desirable additive
system uses a mixture of aromatic or aliphatic quaternary amines,
polysulfide compounds, polyimines and polyethers. Other additives
include metaloids such as selenium, tellurium and sulfur
compounds.
Electrolysis conditions such as electric current concentration,
applied voltage, electric current density, and electrolyte
temperature may be essentially the same as those in conventional
electrolytic copper plating methods. For example, the bath
temperature is typically about room temperature such as about
20-27.degree. C. but may be at elevated temperatures up to about
40.degree. C. or higher. The current density is typically up to
about 100 amps per square foot (ASF) typically about 15 to 40 ASF.
It is preferred to use an anode to cathode ratio of about 2:1, but
this may also vary widely from about 1:1 to 4:1. The method of the
invention also uses mixing in the electroplating tank which may be
supplied by agitation or preferably by the circulating flow of
recycle electrolyte through the tank. In the preferred apparatus of
the invention as shown in the Figures, the flow through the
electroplating tank provides a residence time of electrolyte in the
tank of less than about 1 minute typically less than 30 seconds,
e.g., 10-20 seconds.
The copper containing solution which is added to the system to
replenish the copper plated onto the substrate and removed from the
system in the removal stream may be of any suitable concentration.
It is preferred that the solution be a copper sulfate solution near
the saturation level of copper sulfate pentahydrate, e.g., about
275 to 325 g/l, which is about 70-80 g/l as copper. As discussed
hereinabove, the copper containing solution also contains in the
amount to be used the amount of additive used during plating plus
the amount removed in the removal stream. Regardless of the
concentration of the copper in the copper containing solution, it
is an important preferred aspect of the invention that a
substantially equal amount of circulating electrolyte be removed
from the system as is added to the system and more preferred that
the amount added be minimized by using, for example, a concentrated
copper containing solution. Using this addition/removal procedure
maintains the electrolyte in a suitable condition for
electroplating the substrate and provides a bath having a long life
and enhanced operating and plating effects.
Various embodiments of the present invention will now be
illustrated by reference to the following specific examples. This
is to be understood, however, that such examples are presented for
purposes of illustration only and the present invention is in no
way to be deemed as limited thereby.
EXAMPLE 1
A copper electroplating working bath of about 1 gallon was prepared
containing 67 g/l copper sulfate pentahydrate (17 g/l Cu.sup.+2),
190 g/l H.sub.2 SO.sub.4 and 8 ml/l additive system. A polished
brass cathode substrate 2 inch.times.3 inch was hung vertically in
the bath. A pair of platinized titanium anodes each 2 inch.times.6
inch (as submerged in the bath) were also hung vertically in the
bath on each side of the cathode with a space of about 3 inch
between the cathode and each anode. The bath was agitated using a
magnetic stirrer. The bath temperature was maintained at about 21
to 27.degree. C. A DC electric current at 25 amps/ft.sup.2 (ASF)
was employed.
A copper containing solution was prepared containing 75 g/l
Cu.sup.+2 (added as copper sulfate pentahydrate) and 235 ml/l of
the same additive system.
After 6 grams of copper was plated from the gallon bath, 100 ml of
the working electrolyte was removed and 100 ml of the copper
containing solution added. This procedure was followed for 1 metal
turnover (the time required to plate the initial copper in the
electrolyte which is about 64 g of copper).
The copper bath was maintained in a material balance as follows.
For every 6 g of copper plating, removal of 100 ml of electrolyte
removes an additional 1.5 g copper from the working bath for a
total of 7.5 g copper removed from the system. 100 ml of copper
containing solution adds about 7.5 g copper to the bath. Likewise,
for every 6 g copper plated, about 22.5 ml of additive is consumed
and about 0.8 ml is removed in the electrolyte removal portion. 100
ml of the copper containing solution contains about 23.5 ml of the
additive. The additive consumption was determined empirically.
Using the above procedure, essentially 100% plating efficiency was
achieved (based on copper weight gain, plating current density and
plating time). The copper concentration was maintained between
about 15-17 g/l during the 1 metal turnover plating cycle. The
total sulfate concentration decreased about 5%. Consistent,
constant bright copper electroplating results were obtained.
EXAMPLE 2
A copper electroplating apparatus (basically as shown in FIG. 1) is
used to electroplate copper on a 8 inch diameter silicon wafer
having trenches and vias. The insoluble anode is about 8 inch
diameter having a central opening for flow of electrolyte
therethrough and is made of platinized titanium. The temperature of
the bath is maintained at about 21-27.degree. C. and the
electrolyte is circulated through the apparatus at between about
15-25 liters/min providing a residence time in the electroplating
tank of about 10-20 seconds. The electroplating tank holds about
4.5 liters and the total apparatus (including the holding tank)
holds about 20 liters. A current density of about 25 ASF is
employed and the copper bath is maintained in material balance by
removing electrolyte from the apparatus and adding a copper
containing solution to the apparatus as in Example 1.
While the present invention has been particularly described, in
conjunction with a specific preferred embodiment, it is evident
that many alternatives, modifications and variations will be
apparent to those skilled in the art in light of the foregoing
description. It is therefore contemplated that the appended claims
will embrace any such alternatives, modifications and variations as
falling within the true scope and spirit of the present
invention.
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